1. Field of the Invention
The present invention relates to a semiconductor device having a circuit constructed by thin-film transistors (hereinafter referred to as xe2x80x9cTFTxe2x80x9d), and a method of manufacturing the semiconductor device. For example, the present invention relates to an electrooptical device represented by a liquid crystal display panel and electronic equipment in which such an electrooptical device is mounted as a part.
In this specification, the semiconductor device means a general device that functions by using semiconductor characteristics, and an electrooptical device, a semiconductor circuit and electronic equipment are defined as a semiconductor device.
2. Description of the Related Art
Recently, much attention has been paid to a technique of forming a thin film transistor (TFT) by using a semiconductor thin film (having a thickness of about several to several hundreds nm) formed on a substrate having an insulating surface. The thin film transistor has been widely applied to electrical devices such as IC, an electrooptical device, etc., and particularly developments to apply the thin film transistor to a switching element of an image display device have been rapidly required.
A liquid crystal display device is well known as an image display device. The active matrix type liquid crystal display device has been more frequently used than the passive type liquid crystal display device because higher-definition images can be provided by the active matrix type liquid crystal display device. In the active matrix type liquid crystal display device, a display pattern is formed on a screen by driving pixel electrodes arranged in a matrix form. More specifically, a voltage is applied across a selected pixel electrode and a counter electrode confronting the selected pixel electrode to optically modulate a liquid crystal layer disposed between the pixel electrode and the counter electrode, so that the optical modulation is recognized as a display pattern by a viewer.
Such active matrix type liquid crystal devices have been widely used in more diverse fields, and not only the large-area design of a screen size, but also high definition, high numerical aperture and high reliability design has been increasingly required. At the same time the enhancement of the productivity and the reduction in manufacturing cost have been also increasingly required.
When TFT is formed by using aluminum as a gate wiring material for TFT, malfunction of TFT and degradation in TFT characteristics are caused by formation of projections such as hillocks, whiskers, etc. due to a heat treatment and diffusion of aluminum atoms in a channel-forming region. On the other hand, when metal material having high resistance to the heat treatment, typically a metal element having a high melting point is used in order to avoid the above problem, there occurs another problem that the resistance of wires would increase if the screen size is increased, resulting in increase of power consumption, etc.
Therefore, an object of the present invention is to provide the structure of a semiconductor device that can reduce power consumption even when the screen size is increased, and method of manufacturing the semiconductor device.
According to the present invention, in order to attain the above object, source wires and gate wires are formed by low-resistance material (typically, aluminum, silver, copper or alloy thereof). The gate electrode is provided on a layer different from that of the gate wires. Further, all the NMOS circuits of a driving circuit are formed by n-channel type TFTs, and TFTs of a pixel portion are also formed of n-channel type TFTs.
In order to form an NMOS circuit by combining n-channel type TFTs, there are two cases, one case where the NMOS circuit is formed by combining enhancement type TFTs as shown in FIG. 8A (hereinafter referred to as xe2x80x9cEEMOS circuitxe2x80x9d), and the other case where the NMOS circuit is formed by combining an enhancement type and a depression type (hereinafter referred to as xe2x80x9cEDMOS circuitxe2x80x9d) as shown in FIG. 8B.
In order to form the enhancement type and the depression type separately from each other, an element belonging to the fifteenth group of the periodic table (preferably phosphorus) or an element belonging to the thirteenth group of the periodic table (preferably boron) may be suitably doped into a semiconductor serving as a channel-forming region.
The source wires of the pixel portion are formed in a step different from that of the source wires of the driving circuit portion.
According to an aspect of the present invention, there is provided a semiconductor device equipped with TFT containing a semiconductor layer formed on an insulating surface, an insulating film formed on the semiconductor layer, and a gate electrode formed on the insulating film, characterized by including: a pixel portion having a first n-channel type TFT and a driving circuit having a circuit comprising a second n-channel type TFT and a third n-channel type TFT, wherein the gate electrode of each of the first n-channel type TFT, the second n-channel type TFT and the third n-channel type TFT has a laminate structure comprising a first conductive layer having a first width as a lower layer and a second conductive layer having a second width smaller than the first width as an upper layer.
According to another aspect of the present invention, there is provided a semiconductor device equipped with TFT containing a semiconductor layer formed on an insulating surface, an insulating film formed on the semiconductor layer, and a gate electrode formed on the insulating film, characterized by including: a pixel portion having a first n-channel type TFT, and a driving circuit having a second n-channel type TFT and a third n-channel type TFT, wherein the gate electrode of the first n-channel type TFT has a laminate structure comprising a second conductive layer and a first conductive layer having the same width as the second conductive layer, and the gate electrode of each of the second and third n-channel type TFTs has a laminate structure comprising a first conductive layer having a first width as a lower layer and a second conductive layer having a second width smaller than the first width as an upper layer.
In each of the above semiconductor devices, an EEMOS circuit or an EDMOS circuit is formed by the second n-channel type TFT and the third n-channel type TFT.
In each of the above semiconductor devices, each of the n-channel type TFTs of the driving circuit has a gate electrode having a tapered portion, a channel-forming region overlapped with the gate electrode and an impurity region partially-overlapped with the gate electrode.
In each of the above semiconductor devices, the concentration of impurities in the impurity region of the n-channel type TFT contains an area having a concentration gradient in the range from at least 1xc3x971017 to 1xc3x971018/cm3, and the concentration of the impurities is increased as the distance from the channel-forming region is increased.
In each of the above semiconductor devices, the source wires of the n-channel type TFTs of the driving circuit and the source wires of the n-channel type TFTs of the pixel portion are formed of different materials.
In each of the above semiconductor devices, the source wires of the pixel portion are formed of materials that mainly contain Al, Cu or Ag.
In each of the above semiconductor devices, the source wires of the pixel portion are formed by a sputtering method, a print method, a plating method or any combination thereof.
Each of the above semiconductor devices is a reflection type or transmission type liquid crystal module.
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device having a driving circuit and a pixel portion on an insulating surface, characterized by comprising: a step of forming a semiconductor layer on an insulating surface; a step of forming a first insulating film on the semiconductor layer; a step of forming a first gate electrode on the first insulating film; a step of doping impurity elements providing n-type into the semiconductor layer by using the first gate electrode as a mask to form an n-type first impurity region; a step of etching the first gate electrode to form a tapered portion; a step of doping impurity elements providing n-type into a semiconductor layer while passing through the taper portion of the first gate electrode to thereby form an n-type second impurity region; a step of forming a second insulating film so as to cover the first gate electrode; a step of forming source wires of the pixel portion on the second insulating film; a step of forming a third insulating film so as to cover the source wires of the pixel portion; and a step of forming a source wire of the driving circuit and a gate wire on the third insulating film.
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device having an n-channel type TFT having a first semiconductor layer and a first gate electrode on an insulating surface, and an n-channel type TFT having a second semiconductor layer and a second gate electrode, characterized by comprising: a step of forming a first semiconductor layer and a second semiconductor layer on an insulating surface; a step of forming a first insulating film on the first semiconductor layer and the second semiconductor layer; a step of forming first gate electrodes on the first insulating film; a step of doping impurity elements providing n-type into the first semiconductor layer and the second semiconductor layer by using the first gate electrodes as masks to form n-type first impurity regions; a step of etching the first gate electrodes to form tapered portions; a step of doping impurity elements providing n-type into the first semiconductor layer and the second semiconductor layer while passing through the tapered portions of the first gate electrodes to form n-type second impurity regions; a step of selectively removing only the tapered portion of the first gate electrode above the second semiconductor layer to form a second gate electrode; a step of forming a second insulating film so as to cover the first gate electrode and the second gate electrode; a step of forming source wires of the pixel portion on the second insulating film; a step of forming a third insulating film so as to cover the source wires of the pixel portion; and a step of forming source wires of the driving circuits and gate wires on the third insulating film.
In the above manufacturing method, the n-channel type TFT having the first gate electrode is a TFT of the driving circuit.
In the above manufacturing method, the n-channel type TFT having the second gate electrode is a TFT of the pixel portion.
In the above manufacturing method, a pixel electrode is formed at the same time as the source wires of the driving circuit.
In the above manufacturing method, the step of forming the source wires of the pixel portion is a sputtering method, a print method, a plating method or a combination thereof.
In the above manufacturing method, the first gate electrode has a laminate structure comprising a first conductive layer having a first width as a lower layer and a second conductive layer having a second width smaller than the first width as an upper layer. The sectional shape of an area of the first conductive layer that is not overlapped with the second conductive layer is a tapered shape.